The present invention relates to a method of forming a pressure resistant end shell for a container. More particularly, this invention relates to a method of forming a reinforcing channel in a generally disc-shaped end shell by a controlled bending or folding operation in which a portion of a substantially planar central wall portion defining the bottom recess of the disc-shaped end shell is raised as the peripheral reinforcing channel is formed therearound.
It has been taught in the prior art that a conventional can end may have its pressure resistance increased by increasing the depth of the annular groove with respect to the central panel and maintaining a tight radius of curvature in the annular groove. Prior art patents disclosing deeper than normally experienced annular grooves with tight radii of curvature for the purpose of increasing pressure holding capabilities, buckle resistance and the like include U.S. Pat. Nos. 4,031,837; 3,417,898 and 3,843,014. In particular, U.S. Pat. No. 4,031,837 teaches a method of reforming a conventional can end by moving a drawing tool into the conventional annular groove while supporting the central panel to draw the metal, and thereby increase the depth of the annular groove with respect to the central panel.
When seamed onto a container, such can ends having relatively deep annular grooves have been found to be able to withstand increased internal pressures without buckling. It has thus become possible to reduce the gauge thickness of the can end about 10 to 20 percent while maintaining internal pressure resistance capabilities of the conventional can end.
It has become apparent that drawing a deeper than conventional annular groove has an overall effect of increasing the pressure holding capabilities of a container even though two dichotomous principles are at work. First, the deepening of the annular groove and the tightening of its radius of curvature act to increase pressure resistance. However, drawing has the effect of thinning the metal which acts to decrease pressure resistance. It follows, logically, that a method of forming a deep annular groove having a tight radius of curvature, without thinning the sheet metal would result in a can end having superior pressure resistant capabilities.
Accordingly, a new and improved method of forming a pressure resistant end shell without thinning of the sheet metal is desired to further increase the pressure holding capabilities of the container to which the end shell is secured or, alternatively, permit further reduction of gauge thickness of the end shell without loss of pressure holding capabilities.